NPL 1 discloses High Efficiency Video Coding (HECV) that is an image coding method based on H.265 standard recommended by ITU-T. ITU-T is an abbreviation of “International Telecommunication Union Telecommunication Standardization Sector”. H.265 is a video compression standard.
In HEVC, each frame constituting a digitalized image is divided into coding tree units (CTUs) and each CTU is coded in the raster scan order. Each CTU has a quadtree structure divided into four coding units (CUs). Each CU is divided into units called prediction units (PUs). Each CU performs motion compensation prediction and image prediction for each divided PU. Prediction error resulting from these predictions performed by each CU is divided into a quadtree of transform units (TUs) and the divided prediction error is frequency-converted. The frequency-converted prediction error is quantized per TU and then coded by an arithmetic coding unit.
The arithmetic coding unit performs coding according to the following procedure. First, configuration information for CU, PU, and TU and post-quantization prediction residual, inputted by indicates of a plurality of signals, are transformed to a symbol sequence by a binarization unit. A symbol takes a value of, for example, 0 or 1. The post-quantization prediction residual is transformed to a symbol sequence (a symbol sequence includes 0 or 1) by a transform technique chosen to be appropriate for the syntax of the symbol sequence.
Next, arithmetic coding is performed by using the inputted symbol sequence, symbol sequences formed according to syntaxes, and contexts determined according to the syntaxes. When a plurality of symbol sequences are formed in accordance with a syntax, various contexts may be generated depending on the symbols included in the symbol sequences. A context includes a pair of a most probable symbol (MPS) and the occurrence probability of the MPS. In HEVC, occurrence probability and changes of occurrence probability are represented by 61 states and state transitions between the states. Context-dependent symbols are called context symbols. Hereinafter, it is assumed that a context symbol indicates a context-dependent symbol.
HEVC also includes special symbols not dependent on a context (independent symbols). Such special symbols are called bypass symbols. The occurrence probability of a bypass symbol is fixed at a predetermined probability (e.g., 50%). There is no need for updating the context for a bypass symbol since the occurrence probability of a bypass symbol is fixed. Therefore, the computational complexity of the coding processing of a bypass symbol is smaller than that of a context symbol. Hereinafter, a bypass symbol indicates a symbol not dependent on a context (an independent symbol).
Examples of arithmetic coding devices are disclosed in NPL 1 and PTL 1.
An arithmetic coding device described in NPL 1 includes a context symbol processing unit configured with engines connected in series for the coding processing of context symbols and capable of processing as many as four context symbols at one time. The arithmetic coding device separates a symbol sequence into context symbols and bypass symbols. The context symbols and the bypass symbols thus separated are respectively coded by separate processing indicates.
PTL 1 discloses a video coding device for generating a code for an input image in accordance with a generation technique of an arithmetic code. The video coding device converts a type of processing applied to pixel values of decoded image into a binary signal and codes the converted binary signal in accordance with a procedure for coding bypass symbols. The pixel values is in an image obtained by decoding a code sequence of the coded input image.